1. Field of the Invention
The present invention is directed to the growth of single crystal structures from a seed crystal and more specifically to an improved seed holder for use with a conventional cylindrical quartz boat during the growth process.
2. Prior Art
Gallium arsenide (GaAs) is a III-V compound semiconducting material whose properties make it of interest for certain applications such as light emitting diodes, laser diodes, Gunn diodes, high temperature rectifiers and the like. In most applications gallium arsenide must be a single crystal with certain electrical and structural properties.
Several methods may be used to grow bulk single crystal gallium arsenide. In the well known Czochralski method and variations thereof, a single crystal seed is caused, by mechanical or magnetic means, to contact the surface of a melt of the material to be crystallized, and slowly withdrawing the seed, usually with rotation, at a rate which permits the melt to freeze progressively onto the seed as a growing ingot of single crystal material.
Other methods for growing single crystal materials wherein seeds have been used for initiating crystal growth include horizontally disposed operations such as the Bridgeman and the gradient freeze techniques.
The temperature gradient freeze method generally consists of placing polycrystalline material in a crucible, melting the polycrystalline material in the crucible and placing the crucible in a tubular furnace which is capable of producing a temperature gradient along its length so that it is hotter at one end than the other. As the temperature of the furnace is reduced and the gradient is shifted, a portion of material within the crucible will freeze causing a solid-liquid interface. Thus, when the gradient has shifted to a point below the freezing point of the material, a single crystal structure is formed within the crucible. Such a method is disclosed in U.S. Pat. No. 3,242,015 to Harris.
The Bridgeman technique is exemplified in U.S. Pat. No. 3,520,810 to Plaskett el al wherein a seed crystal is disposed in one end of a quartz boat having a supply of polycrystalline material such as gallium in the other end of the boat. The boat is placed in an ampoule along with a supply of arsenic adjacent the end of the boat having the seed therein. The ampoule is evacuated, sealed and partially inserted into a furnace to first melt the polycrystalline material. The gallium is permitted to melt and in doing so, reacts with the arsenic to form a gallium arsenide melt which will contact the exposed end face of the seed crystal. The ampoule is only partially inserted into the furnace so that upon programmed heating of the ampoule, the arsenic vaporizes and reacts with molten gallium to form gallium arsenide and provide the necessary arsenic vapor over the melt to stabilize and equilibrate the melt. However, such a method was generally unsuccessful since the molten gallium and unequilibrated gallium arsenide were attacking the seed crystal, dissolving arsenic atoms therefrom, melting the gallium atoms and nucleating polycrystalline growth.
In order to overcome the foregoing difficulties, it has now been proposed to locate the seed end of a crucible or boat at an elevated position within the furnace to prevent the molten gallium and unequilibrated gallium from initially contacting the seed crystal by providing a gradual incline for the bottom surface of the boat. In operation, when the more volatile component of a compound to be formed is vaporized and begins to react with the molten less volatile components, the increasing volume of unsaturated melt moves up the inclined bottom of the crucible toward the seed. By the time the melt has reached the seed crystal, the compound has been formed and equilibrated so that upon subsequent cooling of the melt, the seeded crystal growth will yield an ingot of single crystal compounds.